Thank you for your effort and feedback!
To be honest I was afraid the game is too easy once the player starts using RMB. It's valuable to hear about your experience I will make sure to take it into account when balancing difficulty.
I will also test for a bug preventing the sixth colour from appearing when it should, If there is one I missed it. On the other hand I do think about adding a design that will better suggest that the colour of tile itself does not count towards unlocking, because it's visually misleading in current state.